This invention relates to a novel optical system which has several inherent advantages over existing monochromators employing electromagnetic radiation and operating at grazing incidence.
Rowland (1883) was the first to design a self-focusing diffraction grating, thereby constructed a reflection grating monochromator consisting of a single element having useable efficiency. The Rowland grating comprises grooves equally spaced along the chord of a concave spherical surface. The spectral images are in focus along a circle whose diameter equals the grating radius of curvature. Monochromators based upon this design require effective movement of at least one of the slits along the Rowland circle during the wavelength scan.
To provide useable reflection efficiency for wavelengths shorter than approximately 1000 .ANG., the grating is generally operated at grazing incidence. In this application, designs based upon the Rowland grating become increasingly cumbersome, due to the fact that the Rowland circle must also lie at a grazing angle relative to the light rays. Thus, the required slit movements become enormous, resulting in complex mechanical designs and large vacuum and mounting structures. In addition, optimal relaying and refocusing of the light is obtained only if the attached target chamber and/or light source chamber is moved in concert with the slit(s). The expense and mechanical awkwardness of such systems prohibits their widespread use as a practical method of achieving high resolution.
Modern grazing incidence embodiments of the Rowland concept (e.g. Brown et al, U.S. Pat. No. 4,398,823) have been adapted to use with fixed beam directions, but only with the introduction of auxiliary mirrors which must undergo complicated motions in concert with the grating scan and translation of one slit along the beam direction. Absent of such undesirable complications, a conventional concave grating can simply be rotated about its pole to select the wavelength diffracted between fixed slits, as demonstrated by the Hettrick et al (1986) "high throughput monochromator." Due to a drastic departure from the Rowland condition, such an optical system is limited to low or moderate spectral resolution as discussed by Hettrick (1988).
Recently, monochromators have been developed which employ diffraction grating designs in which the grating surface comprises groove elements which are spaced from one another by systematically varying distances. Such monochromators can exhibit improved performance compared to those which employ conventional equally spaced gratings, due to the extra degree of freedom delivered by a judicious choice of the variation in groove spacings. In this way, aberrations in the image may be reduced or eliminated at one or more wavelengths, resulting in higher spectral and/or spatial resolution.
Prior art designs employing this idea have achieved wavelength scanning in one of two ways, through either pure rotation of the grating or through pure translation of the grating. At normal or near normal incidence (e.g. Seya-Namioka mount), a given varied spacing on a concave surface can maintain an improvement in the resolution over a broad wavelength region as the grating is purely rotated (Harada et al, U.S. Pat. No. 4,312,569). However, at grazing incidence no net improvement is obtained in this manner. Therefore, prior art grazing incidence varied-space designs which scan wavelength through pure rotation of the grating have utilized a plane (or large radius) grating in combination with an auxiliary mirror. This mirror has either been flat and undergone a complicated scanning motion (Harada et al, 1984) or be highly figured to provide focusing in the dispersion direction of the grating (Hettrick et al, U.S. Pat. No. 4,776,696; Hettrick 1988; Hettrick et al 1988; Pouey, U.S. Pat. No. 4,241,999). Any such auxiliary mirrors decrease the efficiency, add to the size and expense of the resulting monochromator, and introduce additional sources of fabrication and alignment errors. The method of pure grating translation (Aspnes, U.S. Pat. No. 4,492,466) employs a long cylindrical grating requiring a variation in groove spacing which is at least as large as the wavelength region which may be scanned, imposing a severe technical limitation on the grating fabrication. In none of the above designs are the images perfectly in focus at all scanned wavelengths, this condition being only approximately met at a linearly increasing level of accuracy as the numerical aperture is reduced.
A monochromator which employs a varied-space diffraction grating which is self-focusing, requires only modest amounts of variation in the groove spacings, produces spectral images which are in focus at all scanned wavelengths, and operates at grazing incidence with fixed slit positions and beam directions, would be a great advance in the field of optics.